In the near future, optical free space communication between satellites as well as between a satellite and a ground station will become an important and, on board of the satellites a weight-saving, complement to the existing microwave technology. So-called optical terminals consist of one or several telescopes, which reduce the angular range of the field of vision of an optical receiver in the direction toward the counter-station and also see to a directional dissemination of the signals to be transmitted. Furthermore, several movable mirrors are provided, by means of which the alignment of the transmitting and receiving directions is performed. Besides the direct detection of the optical output of the transmitter of the counter-station as the transmission method, the coherent superimposition of the received light with the light of the same frequency of a local oscillator laser plays an important role, since in the process the indifference to interferences by radiation in the background is advantageous, besides a great sensitivity to the signal to be detected.
The output of the light signal to be detected is in general very low with the above mentioned systems, accordingly it is intended to use the largest possible portion of the light signal in a receiver for detecting the transmitted data. For example, because of the narrowband character of an error signal from the alignment device of the optical portion of the receiver, it would be conceivable to conduct a very small portion of the light entering the receiver to a field of detector means, which are delimited in respect to each other and are arranged in a plane, in order to obtain a directional error signal by means of the detection of the respectively illuminated detector means. However, such a method is only used for the rough alignment during the establishment of the connection. Since the light sources used for data transmission operate at a considerably lower optical output than special optical beacon transmitters used for establishing the connection, a coherent detection method is needed, which requires additional light output from a laser oscillator provided for the superimposition of the received light signal, as well as additional electronic means.